Have you ever bought a house before? There's so many people involved with the transaction, and the forms are so detailed, and every little thing is checked to the Nth degree, that everyone gets to see every detail of your personal finances.
You can't pay for that sort of thing with currency bills, the people involved in those kinds of transactions don't handle cash. And to get a cashier's check, you gotta go to bank, and banks are required by law to fill out a large currency report on anything over a certain amount (which is surprisingly low).
Sorry, there's no way around it. You gotta launder that drug money properly.